The invention relates to a method for operating a spark-ignition, direct-injection internal combustion engine based in particular on the 4-stroke principle. The invention also relates to a method for the cold-running operation of a spark-ignition, direct-injection internal combustion engine which operates in particular on the 4-stroke principle.
In motor vehicles, in particular in passenger vehicles, use is widely made of spark-ignition internal combustion engines which are also referred to as Otto engines. Internal combustion engines of said type are increasingly provided with injectors, by which fuel is directly injected into the cylinders of the internal combustion engine. With fresh air which is sucked into the cylinders, a fuel/air mixture is generated which is ignited at a predefinable ignition time by means of a spark plug in order to initiate a combustion.
In conventional operating methods of internal combustion engines of said type, a distinction is made between two main operating modes. At high loads, the engine is operated in the so-called homogeneous mode, in which the entire fuel mass is injected during the intake stroke of the engine and is distributed uniformly in the combustion chamber until the ignition time. The same fuel/air ratio prevails in the entire combustion chamber, which fuel/air ratio should, for reasons of exhaust-gas purification, be at least approximately stoichiometric.
At low loads, direct-injection Otto engines can be operated in the so-called stratified mode. In a direct-injection Otto engine with a jet-controlled combustion process, the entire fuel mass is injected immediately before the ignition time in order to generate a defined injection jet in the region of the spark plug. At higher loads, however, the particle emissions in the stratified mode increase drastically, since the fresh mixture in the region of the spark plug becomes very rich, although the fuel/air ratio averaged over the entire cylinder volume remains considerably lean. The so-called soot limit, that is to say the load at which the particle emissions of the engine become unacceptably high, creates a limitation, in the direction of high loads, of the load range which is possible in the stratified mode in the direct-injection Otto engine.
Since, in the stratified mode, the engine is operated with a very high excess of air, very low exhaust-gas temperatures can be generated at low loads. This leads to problems in the usual exhaust-gas purification methods with oxidation or 3-way catalytic converters, since the catalytic converter temperature can fall below the limit temperature of the catalytic converter, also referred to as the light-off temperature, below which the catalytic converter is no longer effective. The catalytic converter is in this case no longer capable of oxidizing carbon monoxide or unburned hydrocarbons to form carbon dioxide. This makes it necessary for measures to be taken to heat the catalytic converter, for example by switching to the homogeneous mode. However, this results in a considerable increase in consumption, and the loss of a part of the fuel saving potential of the direct-injection Otto engine.
In addition, in order to save fuel and in order to reduce pollutant emissions, an operating mode of the internal combustion engine with a high excess of air or with a high content of inert gas is sought. The latter is for example obtained by means of exhaust-gas retention or exhaust-gas recirculation into the cylinder interior space. In this way, the engine can be operated with higher charge masses or higher intake pipe pressures, which leads to a reduction in throttling losses in the homogeneous mode and therefore to a reduction in fuel consumption. By means of a high inert gas content in the combustion chamber, the combustion chamber temperature can be reduced and therefore the formation of nitrogen oxides during the combustion can be reduced. The range in which the engine can be operated with high inert gas contents is however limited by the running smoothness, which decreases drastically in the conventional homogeneous mode with increasing inert gas proportion.
A further problem in conventional operating methods of an Otto engine lies in the cold start and the subsequent cold-running phase. Direct-injection Otto engines are operated in the homogeneous mode in particular at low temperatures below freezing. Since the combustion chamber walls are still very cold after the cold start, large quantities of the fuel which is injected in the intake stroke accumulate on the combustion chamber walls and on the piston head. Said fuel which is not encompassed by the combustion leads to large quantities of unburned hydrocarbons and particle emissions.
A coordinated, operating-point-dependent switch between the previously known homogeneous and stratified injection methods leads, in the switchover phase, to further efficiency losses or to an increase in pollutant emissions.
It is the object of the invention to provide a method for operating a spark-ignition, direct-injection internal combustion engine with reduced fuel consumption and reduced pollutant emissions over a widened range.
It is a further object of the invention to provide for the cold-running operation of a spark-ignition internal combustion engine in which, in connection with good running smoothness and low emissions values, fast heating of the exhaust-gas catalytic converter is achieved.